The invention relates to a tap having a tapered thread portion whose thread teeth located in the tapered section are provided with a relief arranged at a clearance angle.
On a tapered thread portion of a tap the width of the teeth looking in the helical direction of the thread, is known to increase so that when a thread is produced, the wider teeth at the back cause much greater friction than the less wide teeth at the front. The cutting moment increases towards the back, i.e. towards the larger diameter. In order to counter this negative phenomenon, the threads in the tapered section are provided with a relief with which is associated a clearance angle. In the prior art, the grinding wheel producing the relief performs the same feed for each tooth and, consequently, produces relief cuts whose clearance angle reduces or decreases towards the larger diameters. As a result, the remedial effect of the "clearance angle" decreases as the diameter increases and, consequently, as the cutting moment increases. In order to decrease the cutting moment and friction towards the back-end larger diameters, some teeth are omitted towards the back. In consequence of this, each remaining tooth has to remove more metal and larger marks tend to occur in the cut thread. It might be possible to keep the tooth width constant towards the back which would involve exceptionally high production costs.